1. Field of the Invention
The present invention relates to electric power control devices of the type using thyristors, and more particularly to an electric power control device having means for improving the commutation characteristics of the thyristor. The invention therefore relates more particularly to an electric power control device in which reverse bias power, i.e., a revese voltage or current, is applied across the anode and the cathode of the thyristor to cause a reverse current to flow in the gate whereby the thyristor is turned off.
2. Description of the Prior Art
It is well known in the art that, to turn off a thyristor, a reverse voltage is applied across the anode and the cathode for a period longer than the turn-off time. To this end, a forced commutation circuit has been used in the inverter or chopper associated with the thyristor circuit. In such a circuit, the turn-off time has been rather prolonged. Nevertheless, no substantial improvements have been achieved with respect to the turn-off time characteristic, especially with thyristors used with high voltage or heavy current. This has necessitated the use of an intricate turn-off means. In addition, a considerable amount of loss has been incidental to prior art turn-off circuitry.
The gate turn off thyristor (briefly, GTO) has been known in the art. To turn off this type of thyristor, a negative current is supplied to the gate. Turn-on and turn-off can be controlled only by the gate signal. The turn-off characteristics of a GTO is such that the turn-off current gain is nor large enough in the neighborhood of the rated value and rapidly decreases with an increase in the anode current. Accordingly, with a thyristor of large capacity, a large reverse gate current is needed for turn-off. Furthermore, the turn-off characteristic cannot be improved except by the sacrifice of turn-on gain and an increase in the holding current. This has hampered the manufacture of GTO's for heavy current or high voltage applications.
A gate turn-on circuit has been used to turn on the thyristor. In such a circuit, the turn-on energy is sypplied from a turn-on power source through a control switch. Generally, the use of an isolating transformer is required because the cathode potential of the thyristor differs from the potential of the turn-on power source. If the turn-on power source is of commercial power, the size of the transformer becomes quite large, with the result that the stray capacitance and the leakage inductance increase, the rise of the turn-on pulse becomes diminished, and the potential variation on the secondary side of the transformer causes a false turn-on signal to be supplied through the stray capacitance to other thyristors. To prevent false turn-on, the prior art has been required to use a sophisticated turn-on circuit.